tg3: fix return value in tg3_get_stats64

When tp->hw_stats is 0, tg3_get_stats64 should display previously
recorded stats. So it returns &tp->net_stats_prev. But the caller,
dev_get_stats, ignores the return value.

Fix this by assigning tp->net_stats_prev to stats and returning stats.

Signed-off-by: Govindarajulu Varadarajan <_govind@gmx.com>
Acked-by: Prashant Sreedharan <prashant@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>